Recently, ceramic filter structures excellent in heat resistance and corrosion resistance are used as filters for dust collection used for environmental protection such as prevention of environmental pollution and recovery of products from high-temperature gases in a wide variety of the fields such as chemical industry, power industry, steel industry and industrial waste disposal. For example, ceramic filter structures are suitably used as filters for dust collection which are used in atmospheres of high temperature and corrosive gas, such as diesel particulate filters (DPF) collecting particulates discharged from diesel engines.
The ceramic filter structure used for these purposes is generally a ceramic filter structure 21 comprising a plurality of cells 23 defined by partition walls 22 having a porous structure to form flow passages of fluid, wherein plugged portions 25 are formed at one open end portions 24a of specified cells 23 and the other open end portions 24b of the remaining cells 23 to allow the partition walls 22 to serve as filtration layers as shown in FIG. 10 (see, for example, JP-A-56-129020).
When a fluid to be treated, for example, an exhaust gas, flows into the cells 23 of the ceramic filter structure 21 from one open end portions 24a, since the other open end portions 24b of the cells 23 are plugged with the plugged portions 25, the gas flowing into the cells 23 passes through the partition walls 22 having a porous structure and flows out from the cells 23, the other open end portions 24b of which are not plugged. In this case, the partition walls 22 act as a filter to collect particulates such as soot contained in the exhaust gas.
The ceramic filter structure 21 which has collected particulates gradually increases in its pressure loss. The increase of the pressure loss is caused by deposition of the collected particulates on the surface of the partition walls 22, and there are two stages in the process of increase of the pressure loss in the ceramic filter structure as shown in FIG. 11. The first stage A comprises an increase of the pressure loss caused by deposition of the particulates in the pore parts of the partition walls having a porous structure and the uneven dent parts on the surface of the partition walls constituting the ceramic filter structure, and the second stage B comprises an increase of the pressure loss caused by complete filling up of the uneven dent parts on the surface of the partition walls with the particulates and the subsequent formation of a film with the particulates on the surface of the partition walls, the thickness of which gradually increases.
The increase of pressure loss of the first stage A is determined by properties of the particulates, arrangement of ceramic particles constituting the ceramic filter structure, shape of the pore parts of the partition walls and shape of uneven dents on the surface of the partition walls. The increase of pressure loss of the second stage B is determined not by the shape of the ceramic filter structure, but by the properties of the particulates and thickness of the film formed by the particulates.